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Updated: June 15, 2025
ARTHOPODS. These segmented animals with "jointed feet," as their name suggests, may be divided in a general way into water breathers and air breathers. The first-named and lower division comprises the class of the CRUSTACEA, arthropods protected by a hard exterior skeleton, or "crust," of which crabs, crayfish, and lobsters are familiar examples.
In the arthropods, the body is formed of a series more or less long of rings set together; motor activity is thus distributed amongst a varying sometimes a considerable number of appendages, each of which has its special function. In the vertebrates, activity is concentrated in two pairs of members only, and these organs perform functions which depend much less strictly on their form.
The gigantic crustaceans called the EURYPTERIDS were also present in this period. The arthropods had now seized upon the land. Centipedes and insects of a low type, the earliest known land animals, have been discovered in strata of this system.
We have seen that the arthropods were the first of all animals to conquer the realm of the air, the earliest insects appearing in the Ordovician. Insects had now become exceedingly abundant, and the Carboniferous forests swarmed with the ancestral types of dragon flies, some with a spread of wing of more than two feet, May flies, crickets, and locusts.
In many insects and other Arthropods which are not parthenogenetic the male has been found to possess fewer chromosomes than the female. The female forms, as in the above cases of parthenogenesis, only gametes of one kind each with N chromosomes, but the male forms gametes of two sorts, one with N chromosomes, the other with N-l or N-2 chromosomes.
A corresponding germinal condition, the two-layered gastrula, occurs transitorily in the embryological history of all the other Metazoa, from the lowest Cnidaria and Vermes up to man. From the common stock of the Helminthes, or simple worms, there develop as independent main branches the four separate stems of the Molluscs, Star-fishes, Arthropods, and Vertebrates.
The animal kingdom consists, first, of a sub-kingdom of animals which possess a spinal column, or backbone, and which are known as vertebrate animals. Such are all beasts, birds, reptiles, and fishes. There is, further, an immense group of arthropods, consisting of all insects, crab-like creatures, hundred-legs and their allies, with spiders, scorpions, tics and mites.
The higher division, that of the air breathers, includes the following classes: spiders, scorpions, centipedes, and insects. THE TRILOBITE. The aquatic arthropods, the Crustacea, culminated before the air breathers; and while none of the latter are found in the Cambrian, the former were the dominant life of the time in numbers, in size, and in the variety of their forms.
To define these powers, we must consider, in the evolution both of the arthropods and the vertebrates, the species which mark the culminating point of each. How is this point to be determined? Here again, to aim at geometrical precision will lead us astray. There is no single simple sign by which we can recognize that one species is more advanced than another on the same line of evolution.
We put the soil samples in the tulgren funnel and observed the arthropods that fell into the beaker under a compound microscope. We also observed some preserved specimens of microarthropods. 15th November: Pounded 100 gms of soil sample and then sieved each soil sample through 5 sieves. Then weighed the soil in each sieve and noted this down. 16th November: Did sieving of soil in the morning.
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